1. The Field of the Invention
This invention relates to a radial multi-block copolymer containing a tapered block. More particularly, this invention relates to a radial block copolymer that includes a polyvinyl aromatic block, a poly(isoprene-vinyl aromatic)tapered block, a polyisoprene block, and a polybutadiene block and therefore can be used as an excellent pressure-sensitive adhesive material, and a preparation method and usages thereof.
2. Related Prior Art
In general, a variety of block copolymers, such as those composed by both polystyrene and polyisoprene are used as a base resin of hot melt adhesives. Among them, the polystyrene-polyisoprene block copolymer having a relative good balance of price and performance is in much wide use to the public.
The early polystyrene-polyisoprene block copolymer is prepared by a linear coupling method in the presence of a bi-functional coupling agent (refer to U.S. Pat. No. 5,143,969). The subsequent block copolymers are those synthesized using a multi-functional coupling agent for the purpose of expanded applications and enhanced properties, including a branched block copolymer using a tri-functional coupling agent (refer to U.S. Pat. No. 5,194,500), and radial block copolymers using a tetra-functional coupling agent (refer to U.S. Pat. Nos. 5,292,819 and 6,534,593).
The methodical diversity from linear block copolymers to branched or radial block copolymers has contributed to the enhanced properties of the block copolymers and the expansion of the applications from dippers and general adhesive tapes to packaging transparent tapes, book linings, book binders, or the like.
The radial block copolymers typically including styrene and isoprene as principal ingredients are excellent in adhesiveness in the aspect of properties but poor in tackiness and melt viscosity relative to molecular weight, resulting in a deterioration of processability.